EDM Electrodes

ABSTRACT

A precision molded electrical discharge machining electrode is made by shaping a preform from granules of carbon and granules of a refractory material selected from the group consisting of tungsten, molybdenum, carbides thereof, and stoichiometric and hyperstoichiometric carbides of the other elements of the groups IVB, VB, and VIB of the Periodic Table of the Elements, the carbon and refractory granules being interconnected in the form of a skeleton at their contiguous points of contact, and infiltrating the preform with copper, silver, or alloys containing those metals.

This invention relates to a process for forming infiltrated electricaldischarge machining electrodes. In another aspect, it relates to aprocess for making copper or silver infiltrated carbon-containingelectrical discharge machining electrodes and to the electrodes so made.In another aspect, it relates to a monolithic skeletal preform useful inmaking such electrodes. In yet a further aspect, this invention relatesto electrical discharge machining electrodes which are precise replicasof an original pattern or master.

Powder metallurgy techniques have been employed to make precision shapedelectrical discharge machining ("EDM") electrodes from infiltratableskeletal preforms of consolidated refractory metal powders which areinfiltrated with lower melting metals such as copper or silver. U.S.Pat. Nos. 3,823,002 and 3,929,476 describe a process for forming eachelectrodes from, for example, tungsten powder which is molded and firedto form a skeletal preform having perceptible necking between thecontiguous smaller tungsten granules, and then infiltrated with moltencopper. The infiltration, step takes place through capillary action("wicking") at ambient pressure, i.e. at zero pressure differentialbetween the exterior and the interior of the skeletal preform.Infiltration under these conditions appears to be due, in part, to theability of molten copper to "wet" the refractory metal powder from whichthe skeletal preform is made. Commercial EDM electrodes made by theabove process are referred to in the art as "molded copper-tungsten"electrodes and are characterized by low shrinkage during processing andclose fidelity of replication between the finished electrodes and theoriginal master from which the electrode mold was formed. Suchelectrodes also have uniform density and uniform electrical properties.Also, such electrodes can be made in complex shapes having high aspectratios, stepped or undercut profiles, and intricate surface detail.Large numbers of such electrodes may be made from a single mold master.

EDM electrodes have also been made from porous carbon bodies which areimpregnated with copper. They are not made using the above-describedpowder molding process. Because copper does not "wet" carbon at ambientpressure, such electrodes have generally been made by forcing moltencopper into the interior of a graphite body under heat and pressure.Such electrodes are referred to in the art as "copper-graphite"electrodes and are characterized by very high EDM cutting rates. Due tothe pressures required to carry out the copper impregnation step andlimitations inherent in the manufacture of a suitable die in which tomount the graphite body during impregnation, such copper-graphiteelectrodes have generally been manufactured in simple shapes (such asbar stock). If copper-graphite electrodes are to be made having acomplex shape with a high aspect ratio, a stepped or undercut profile,or intricate surface detail, it is ordinarily necessary to createelectrodes having such a complex shape by machining each electrode froma single shape (such as bar stock) to the desired complex shape. Thisprocess involves waste of materials and requires precision machining ofeach electrode. Also, the machining process "disturbs" the copper andgraphite at the surface of the electrode, resulting in an electrode witha surface microstructure which differs from the microstructure withinthe body of the electrode.

Other references disclose methods for infiltrating or impregnating acarbon-containing body with copper. For example, in U.S. Pat. No.3,549,408, carbon is treated with boric acid or ammonium phosphatefollowed by copper infiltration under pressure. Also, U.S. Pat. Nos.3,235,346 and 3,348,967 describe a method for infiltrating carbon byalloying an infiltrant such as copper with carbide forming metals.Because these methods utilize applied pressure, surface treatment of thecarbon-containing preform, or introduction into the carbon body ofmaterials which react with carbon, they could tend to promotedimensional changes in the carbon body during infiltration and therebybe unsuitable to the manufacture of a precision EDM electrode with acomplex shape.

Other references have described copper or silver infiltrated compositestructures which contain carbon. For example, U.S. Pat. No. 4,153,755discloses a method for preparing electrical contacts from compositescontaining tungsten, graphite, silver or copper, and a wetting promotingmetal such as iron, cobalt, or nickel. The steps of that method involvemixing powdered tungsten, powdered silver or copper, the wettingpromoting metal, and graphite, compacting the mixture in a press,sintering and then cooling the compacted mass, granulating the sintered,cooled mass and admixing it with additional graphite, pressing theresulting mixture into a porous form, and impregnating the porous partwith silver or copper. That patent discloses that without the use of awetting promoting metal and a two-step graphite addition, the pressed,impregnated contact has a "bothersome" residual porosity and most of thegraphite in the contact reacts with the tungsten to form tungstencarbide (see col. 2, lines 11 to 65). The wetting promoting metals usedin the method of that patent (iron, cobalt and nickel) all dissolve incopper or silver and thereupon react with carbon. Such reactions wouldlead to dimensional changes in the porous part during infiltration(e.g., shrinkage) and would make the method of that patent unsuited toaccurate replication of precision EDM electrodes having complex shapes.

U.S. Pat. No. 2,289,708 describes an automatic electric circuit breakercontact set, one contact of which is a composite formed of silver,tungsten, and approximately 1 percent by weight of carbon (equivalent toup to about 6 to 7 volume percent). These contacts are said to havereduced contact resistance and less tendency to weld together. If such acontact were prepared having a thickness greater than about 1.5 mm, thecontact would have non-uniform density due to the pressing operation bywhich it was formed. The pressing operation of that patent would notordinarily be useful in forming EDM electrodes having complex shapes andrequiring uniform EDM cutting performance.

It is an object of the present invention to achieve essentially completeinfiltration of a precision shaped carbon-containing EDM electrode butwithout the need for applied pressure infiltration, conventional surfacetreatment of the carbon, the use of wetting-promoting metals which reactwith carbon, or alloying of the infiltrant with other metals. Anotherobject of the invention is to provide a process for providing accuratelyreplicated EDM electrodes at low cost. An additional object of theinvention is the formation of EDM electrodes with good dimensionalstability. A further object of the invention is the formation of EDMelectrodes with high cutting rates and low wear rates.

The present invention provides in one aspect an electrical dischargemachining electrode consisting essentially of:

(1) a monolithic skeleton consisting essentially of (a) granules ofcarbon and (b) granules of a refractory material selected from the groupconsisting of tungsten, molybdenum, carbides of these two elements, andstoichiometric and hyperstoichiometric carbides of the other elements ofgroups IVB, VB, and VIB of the Periodic Table of the Elements, themajority of said carbon granules being greater than about one micrometerin mean diameter, said refractory material being wettable by moltencopper or silver, said carbon granules and said refractory granulesbeing interconnected at their contiguous points of contact; and

(2) a continuous phase consisting essentially of copper, silver, oralloys containing copper or silver, and occupying the connected porosityin said skeleton;

wherein the volume percent of said carbon, said refractory, and saidcontinuous phase are on or within the boundaries DEFG of FIG. 7, and thefraction expressed by the Formula ##EQU1## wherein C is said carbon, Ris said refractory, the volume percent terms are the fraction of saidarticle occupied by C or R, and the surface area terms are the surfaceareas in m² /g of said carbon or refractory granules measured beforesaid carbon or refractory granules are used in making said article, isless than about 75, said electrode thereby being a monolithic structureconsisting essentially of two intermeshed matrices, having homogeneousdensity, being substantially free of voids, and being substantially freeof materials which are soluble in said continuous phase and reactivewith carbon.

In the accompanying drawing,

FIG. 1 is a flow diagram illustrating a process of the present inventionfor making the EDM electrodes thereof;

FIG. 2 is a view in perspective of an EDM electrode of the presentinvention;

FIG. 3 is a photomicrograph of an uninfiltrated skeletal preform of thisinvention;

FIG. 4, FIG. 5 and FIG. 6 are photomicrographs at various magnificationsof an infiltrated EDM electrode of this invention; and

FIG. 7 is a ternary diagram illustrating compositions of the invention.

In the practice of this invention, a replicating master in the desiredshape is used to prepare a flexible rubber mold. Next, carbon granules(generally in the form of amorphous carbon or graphite granules orpowder and hereafter referred to generally as "carbon") are mixed withcertain granules of refractory material (viz., tungsten, molybdenum,carbides of either, and stoichiometric and hyperstoichiometric carbidesof the other elements of groups IVB, VB, and VIB of the Periodic Tableof the Elements), said refractory material, when in its solid form beingwettable by copper, silver, or alloys containing copper or silver."Stoichiometric" carbides, as used herein, are carbides with astoichiometric combination of metal and carbon. "Hyperstoichiometric"carbides, as used herein, are carbides containing an amount of carbon inexcess of the amount of carbon in said stoichiometric carbides (incontrast to "hypostoichiometric" carbides, which contain an amount ofmetal in excess of the amount of metal in said stoichiometric carbides).The mixture of carbon and refractory granules is mixed with a heatfugitive organic binder and shaped and heated to form a monolithic,infiltratable, skeletal preform having the same shape as the ultimateEDM electrode. The preform is then infiltrated with molten metal (viz.,copper, silver, or alloys containing these elemental metals) at ambientpressure, i.e. at a zero pressure differential between the exterior andthe interior of the skeletal preform (such ambient pressure infiltrationbeing referred to hereafter as "infiltration").

The replicating master used to prepare precision shaped EDM electrodesaccording to the present invention can be made from wood, plastic,metal, or other machinable or formable material. If a finished electrodeprepared according to the process of the present invention exhibitsdimensional change (e.g. shrinkage), then the dimensions of thereplicating master can be adjusted (e.g. made larger) to compensate forthose dimensional changes occurring during processing.

The molding materials which can be used to prepare a flexible mold inthe process of this invention are those which cure to an elastic orflexible rubbery form and generally have a Shore A durometer value ofabout 25-60, and reproduce the fine details of the replicating masterwithout significant dimensional, e.g. without more than 1 percent linearchange from the replicating master. The molding materials should not bedegraded when heated to molding temperatures, e.g. 180° C., and shouldhave a low cure temperature, e.g. room temperature. A low temperaturecuring molding material will form a mold which maintains closedimensional control from master to mold. A high temperature curingmolding material will generally produce a mold having dimensionssubstantially different from those of the master. To maintaindimensional control, it is preferable that the mold material are curablesilicone rubbers, such as those described in Bulletin "RTV" 08-347 ofJanuary, 1969, of the Dow Corning Co., and low exotherm urethane resins.Such molding materials cure to an elastic or rubbery form having a lowpost cure shrinkage. The molding material can be reinforced by theaddition of about 30 volume percent of less than 44 micrometer glassbeads in order to improve dimensional control in the molding process.

The amount of molding material used to form a mold of the replicatingmaster can very depending on the particular molding material used andthe shape of the replicating master. It has been found that about 10-14cubic centimeters of molding material for each cubic centimeter of thereplicating master will form a mold which retains the desired flexibleproperties and also has sufficient strength to support the smallhydrostatic head produced by the warm powder-binder mixture in the moldbefore solidification of the binder.

The molding conditions, hereinafter discussed, for molding the EDMelectrodes of this invention permit the use of an inexpensive soft,elastic or rubbery mold because the only pressure applied is thehydrostatic head of the warm powder-binder mixture in the mold, whichpressure is very small and causes negligible distortion. The mildmolding conditions thus help ensure a precisely molded green preformeven though a highly deformable mold is used. In addition, the moldingtechnique results in a molded green preform with a uniform density.Uniform density of the molded green preform helps prevent unevendimensional change during infiltration.

The carbon granules can be used in the form of free flowing, discretebeads, spheres, flakes, equiaxed particles, or agglomerates thereof,with irregular or smooth surfaces. Equiaxed graphite particles havingless than about 200 micrometer diameter and having less than about 15 m²/g surface area are preferably used, since they are electricallyconductive, inexpensive, and can be readily used to make precisionshaped EDM electrodes according to the present invention. Thedistribution of sizes of the individual carbon granules is dependentupon the performance requirements of the EDM electrode. For example, theparticle size distribution and shape of the carbon granules greatlyaffect the packing efficiency of the skeletal matrix. Higher packingefficiency generally leads to lower shrinkage during infiltration,thereby aiding in the accurate replication of complex electrode shapesfrom a master or pattern shape. Higher packing efficiency generallyyields an EDM electrode which in use has higher metal removal rate andgenerally minimal wear. Also, use of small carbon granules in an EDMelectrode generally leads to improved (i.e. finer) workpiece surfacefinish and increased metal removal rate, but at an increase in end andcorner electrode wear. Use of large carbon granules in the EDMelectrodes of the present invention generally leads to a rougherworkpiece finish and decreased metal removal rate, but decreased end andcorner electrode wear.

The carbon volume percent is high enough (e.g. about 5 volume percent orgreater) to give improved EDM burn rate performance compared to an EDMelectrode made without carbon granules. At levels around 5 volumepercent of 7 to 50 micrometer carbon granules, EDM electrodes of thepresent invention exhibit increased rough mode cutting rates andnoticeably smoother burning performance (i.e. the electrode operateswith less hesitation and less arcing of the electrode occurs) comparedto a copper-tungsten EDM electrode made without carbon granules.

For high EDM cutting rates, a large number of carbon granules should bepresent. This can be achieved by using very fine carbon particles and/orby using a high volume percent of carbon. In general, the carbon volumepercent in an EDM electrode of the present invention should bepreferably about 10 volume percent or greater, more preferably about 15volume percent or greater, and most preferably about 20 to 30 volumepercent. Also, as the ratio of the amount of carbon to refractory isincreased, roughing mode metal removal rate of the electrode willincrease but the workpiece will exhibit a rougher surface finish andwearing of the end and corners of the electrode will increase.

The surface area of the carbon granules used also affects the ease withwhich the carbon-containing skeletal preform may be infiltrated. As thesurface area of carbon within the preform increases, a point will bereached at which one can not carry out infiltration of a preform withcopper, silver, or alloys containing copper or silver. In general, smallcarbon granules have a larger surface area than large carbon granules.However, commercially available carbon granules sometimes exhibit widelyvarying surface areas among samples of similar particle size anddistribution. It is preferred that the carbon granules employed have asmall surface area (as measured by nitrogen adsorption before theintroduction of the granules into a preform), preferably less than about15 m² /g, and more preferably less than about 7 m² /g. Infiltration ofthe shaped preform with copper, silver or their alloys is carried out inobservance of the above formula I: ##EQU2## wherein C is carbon, R isrefractory, the volume percent terms are the fraction of the finalshaped electrode occupied by C or R, and the surface area terms are thesurface areas (in m² /g) of the carbon or refractory granules to be usedin making the preform. Through observance of the above relationship,shaped preforms containing very high loadings of carbon (viz., greaterthan 20 volume percent) can be infiltrated.

If surface area data are unavailable, then mean particle diameter datamay be used as an approximate guide to determine whether infiltrationwill occur. For example, if graphite granules and unimodal refractorygranules are mixed, and the graphite granules have a mean particlediameter of about 25 micrometers, then the ratio of mean particlediameters of graphite granules to refractory granules should be about0.5 to 1 or greater at low carbon volume loadings (e.g. a volume percentratio of carbon to refractory of 1:9), and about 3.5 to 1 or greater atvery high carbon volume loadings (e.g. a volume percent ratio of carbonto refractory of 7:3). Also, if graphite granules and unimodalrefractory granules are mixed, and the graphite granules have a meanparticle diameter of about 100 micrometers, then the ratio of meanparticle diameters of graphite granules to refractory granules should beabout 1 to 1 or greater at low carbon volume loadings, and about 14 to 1or greater at very high carbon volume loadings. However, the above testsbased upon mean particle diameter are less reliable than a test basedupon actual surface area limits, and should be used only if surface areadata is unavailable or would be inconvenient to obtain.

Several commercial powdered carbons useful in the present invention areset out below in Table I.

                  TABLE I                                                         ______________________________________                                                                    Surface                                                                       Area**,                                                             Size*, μm                                                                            m.sup.2 /g                                        ______________________________________                                        "Versar" amorphous carbon beads                                               (Versar Carbon Co.) 30-80       6.56                                          "1264" Equiaxed graphite particles                                            (Asbury Graphite Mills)                                                                           40-200      2.85                                          "4234" Equiaxed graphite particles                                            (Asbury Graphite Mills)                                                                           52-690      1.82                                          "4349" Equiaxed graphite particles                                            (Asbury Graphite Mills)                                                                           10-200      7.2                                           "A-200" Equiaxed graphite particles                                           (Union Carbide Co. of Canada)                                                                     7-50        6.72                                          "KS-5" Equiaxed graphite particles                                            (Dixon Carbon Co.)  1-10        11.6                                          "Statex MT bead" amorphous carbon                                             granules and agglomerates                                                     (Cites Service Co.) <20         9.3                                                               (agglomerates)                                            ______________________________________                                         *Size listed represents a range within which the central 90% of the sampl     particles lie, as measured by Coulter Counter.                                **As measured by nitrogen adsorption.                                    

The granules of refractory material can be tungsten, molybdenum,carbides of either, stoichiometric and hyperstoichiometric carbides ofthe other elements of groups IVB, VB, and VIB of the Periodic Table ofthe Elements, or mixtures thereof, with molybdenum, tungsten, molybdenumcarbide, and tungsten carbide being preferred. The refractory granulesmay also be molybdenum precipitated onto a carbon nucleating agentaccording to the method of U.S. Pat. No. 3,241,949 (such precipitatedmolybdenum being included hereafter in the term "molybdenum" unlessotherwise stated). The refractory granules can be used in the form ofdiscrete beads, spheres, flakes, needles, or equiaxed particles.Preferably the refractory granules are less than about 200 micrometersin mean diameter. The refractory material should be wettable by moltencopper or silver, that is, the refractory should have a sessile droptest wetting angle value of ninety degrees or less under a hydrogenatmosphere. The above test is described, for example, in "Wetting ofCeramic Oxides by Molten Metals under Ultra High Vacuum", F. L. Hardingand D. R. Rossington, J. Am. Cer. Soc. 53, 2, 87-90 (1970) and in "TheWetting of TaC by Liquid Cu and Liquid Ag", S. K. Rhee, J. Am. Cer. Soc.55, 3, 157-159 (1972).

The refractory material should also be essentially insoluble in theinfiltrant metal at infiltration temperatures (e.g. temperatures ofabout 1150° C. for copper), as this will prevent or minimize theoccurrence of solid solution reactions between the refractory materialand carbon granules within the skeletal preform and thereby minimizedimensional changes in the EDM electrode during infiltration. Someportion of the carbon in an EDM electrode of the present invention maybe combined with refractory metal to form refractory carbide. Thiseffect may be pronounced with very small refractory particles (e.g. 1micrometer tungsten).

From the above it may be seen that the chosen refractory is wettable bythe infiltrant but is not soluble in the infiltrant (and minimallyreactive or unreactive with the carbon in the preform under infiltrationconditions). In general, a refractory material which is soluble inmolten copper or silver will also be wettable by such a melt. However,the converse is not true--although many refractory materials which arewettable by molten copper or silver are also soluble in such a melt, afew are not. Refractory materials which are useful in the presentinvention are wettable by molten copper or silver but not soluble insuch a melt and include tungsten, molybdenum, carbides of these twoelements, and stoichiometric and hyperstoichiometric carbides of theother elements of groups IVB, VB, and VIB of the Periodic Table of theElements. Several suitable commercial refractory granules are set outbelow in Table II.

                  TABLE II                                                        ______________________________________                                                                         Sur-                                                          Mean            face                                                          diameter,                                                                            Size*,   area**,                                                       μm  μm    m.sup.2 /g                                   ______________________________________                                        Tungsten (General Electric Co.)                                                                  0.8      0.7-14   1.45                                     Tungsten (General Electric Co.)                                                                  1.5      0.9-15   0.55                                     Tungsten (GTE Sylvania Inc.)                                                                     8        6-19     0.10                                     Tungsten (General Electric Co.)                                                                  8        6-19     0.12                                     Tungsten (General Electric Co.)                                                                  15       8-74     0.05                                     "M-70" Tungsten (GTE                                                          Sylvania Inc.)     --       40-200   0.01                                     Molybdenum                                                                    (General Electric Co.)                                                                           4-6      --       0.33                                     "SD-252" Molybdenum                                                           0.8 m particles and agglomerates                                              (GTE Sylvania Inc.)                                                                              <8       --       0.44                                                        (agglom-                                                                      erates)                                                    "SC-60" Tungsten carbide                                                      (General Electric Co.)                                                                           --       4-35     0.18                                     "WCOO-PL" Tungsten carbide                                                    (Wah-Chang Div. of                                                            Teledyne Corp.)    1.1      0.3-5.0  1.47                                     Molybdenum carbide                                                            (Ventron Corp.)    4.0      1.4-11.1 0.8                                      Molybdenum coated carbon                                                      (Sherritt-Gordon Mines Ltd.)                                                  50/50 wt. %        --       10-50    2.16                                     Molybdenum coated carbon                                                      (Sherritt-Gordon Mines Ltd.)                                                  75/25 wt. %        --       10-50    2.70                                     ______________________________________                                         *Size listed represents a range within the central 90% of the sample          particles lie, as measured by Coulter Counter.                                **As measured by nitrogen adsorption.                                    

Mixtures of two size distributions of refractory granules (hereafterbimodal mixtures) with a ratio of mean particle sizes of approximately7-10 to 1 give better packing efficency in a carbon-containing preform(i.e., lower interstitial void space and therefore lower infiltrantconcentration) and are preferred. Multimodal mixtures containing threeor more size distributions of refractory granules can also be used. Forexample, 55 volume percent packing efficiency can be obtained in apreform when graphite granules less than 74 micrometers in size arecombined with an equal volume of a bimodal mixture of 65 wt.%, 15micrometer mean diameter tungsten granules and 35 wt.%, 1.5 micrometermean diameter tungsten granules. In contrast, substitution of unimodal1.5 micrometer mean diameter tungsten granules in place of the bimodalmixture in the above preform yields only 50 volume percent packingefficiency. Also, increasing the volume percent of fine refractoryparticles relative to coarse refractory particles tends to improveworkpiece surface finish. For example, an EDM electrode containing abimodal mixture of 65 wt.% 15 micrometer mean diameter tungsten granulesand 35 wt.% 1.5 micrometer mean diameter tungsten granules achievesbetter workpiece surface finish than an EDM electrode in which therelative portions of 15 and 1.5 micrometer tungsten particles are 80%and 20%, respectively.

The carbon and refractory granules are combined by dry mixing, forexample, in a V-blender. The proportions of each of these two componentsaffect the cost and electrical performance of the electrode. Sincecarbon is relatively inexpensive, increasing the carbon proportion inthe preform will lower the cost and improve the rough mode cuttingperformance of the electrode. However, increasing the refractoryproportion in the preform will generally improve workpiece surfacefinish and reduce electrode end and corner wear.

Other materials which do not interfere with the desired electrical ormechanical properties of the electrode can also be added to the carbonand refractory granules. For example, small amounts of oxides may beadded to enhance burn rate or reduce electrode wear. Also, small amounts(viz., about 5 to 10 volume percent) of powdered infiltrant metal orinfiltrant metal oxide (such infiltrant metal oxide being referred tohereafter as "infiltrant metal") may be added to the carbon andrefractory granules. The particle size and amount of such a powderedinfiltrant metal addition should be such as to avoid destroyingcontinuity of the skeleton of carbon and refractory granules. Additionof powdered infiltrant metal can lower the cost of the finished EDMelectrode, cause a slight decrease in roughness of the workpiece, andcause a slight increase in electrode roughing mode removal rate. Thepowdered infiltrant metal melts during infiltration and becomes a partof the continuous infiltrant phase, without destroying the integrity ofthe skeleton of carbon and refractory granules or unduly impairing thedimensional stability of the finished EDM electrode.

The method of forming the electrode involves mixing carbon granules andrefractory granules (and any other desired additional materials such asoxides or powdered infiltrant metal) with a heat fugitive organicbinder, molding the powder-binder mixture, curing the mold contents,removing the bulk of the binder by heating the molded shape, therebyforming a skeletal preform, and infiltrating the preform with moltenmetal. Referring to FIG. 1, a replicating master 101 is used to mold 102a flexible form in the desired shape by surrounding the master with anelastic, rubbery molding compound, e.g. "RTV-J" silicone rubber(commercially available from Dow Corning Co.) and demolding 103 themaster from the cured, solid rubbery mold 104. An admixture of carbon105 and refractory granules 106 is blended 107 to form a powder mixture108 which is next combined with a heat fugitive thermoplastic orthermosetting binder 109 by warm mixing 110 (without causing prematurecure of the binder if a thermosetting binder is used) in a blendingdevice, e.g. a sigma blade mixer, resulting in formation of a warmpowder-binder mixture 111. The carbon and refractory granules areuniformly dispersed in the binder matrix, conducive to forming a preformwith homogeneous (i.e. uniform) density which will be essentiallyuniformly porous when the binder is thermally degraded. Use of carbonshaving a large particle size results in a powder-binder mixture withbetter flow properties (i.e. lower viscosity) than a powder-bindermixture made with carbons having a small particle size. The binder canbe, for example, paraffin, "Emerest 2642" (a polyethylene glycoldistearate with a number average molecular weight of 400), or an aminecured epoxy. Preferably during subsequent heating of the preform, thechosen binder degrades or decomposes at a low temperature and leaves aminimal carbonaceous residue. Thermoplastic binders generally leavelower carbonaceous residues than thermoset binders and have better flowproperties when mixed with high surface area carbon granules. However,use of a thermoset binder yields a preform with a higher green strengthand may offer production advantages. If powdered infiltrant metal isadded to a mixture of carbon, refractory granules, and thermoset binder,then a sufficiently potent curing agent should be used to ensure thatthe preform will cure despite the inhibiting effect such powderedinfiltrant metal may have on some thermoset curing agents.

The flexible mold 104 is heated 114 and the warm powder-binder mixture111 fed directly to the heated mold 115. Optionally, instead ofimmediately molding the warm powder-binder mixture, a mixture made witha thermoplastic binder can be cooled 116 to a solidified mass 117 andmilled 118, preferably in a vacuum, to a granular or free-flowingconsistency ("pill dust" 119) for easy handling and storage, andsubsequently heated 120 to a heated mass 121 at the time of the moldingstep. The heated mold and its contents (the warm powder-binder mixture111 or heated mass 121) are vibrated under vacuum 125 in order to degasthe mixture. The mold contents are allowed to cool 126 and harden. Themolded granule-binder shape is then demolded 127 by applying a vacuum tothe outer walls of the flexible mold. After demolding, the resultant"green" molded preform 128 is a faithful replica of the dimensions ofthe master. This molded shape has good green strength and homogeneousdensity due to the hardened matrix of binder which holds the carbon andrefractory granules in a skeletal shape.

Homogeneous density of the green molded preform is important in thesubsequent heating and infiltration steps. A homogeneous density willminimize or prevent shape distortion when the green molded preform isheated and infiltrated. Also, a homogeneous density will minimize orprevent the formation of localized pockets of infiltrant metal whichotherwise would make the ultimate finished EDM electrode exhibitunstable and nonuniform electrical, mechanical, or physical properties.

The green molded preform 128 is packed in a nonreactive refractorypowder, e.g. alumina or silica, to prevent sagging or loss of dimension,and subsequently heated 130 in a furnace to a temperature of about 780°C. to thermally degrade the binder. The bulk of the binder is therebyremoved from the article as gaseous products of combustion, leaving anamorphous carbonaceous residue which tacks the carbon and refractorygranules together. The carbon, refractory granules, and carbonaceousresidue form a rigid, handleable, skeletal preform 131. The carbon andrefractory granules (and any granules of added materials such as oxidesor powdered infiltrant metal) are in contiguous relationship. They areinterconnected or adhered together due to the cementing action of thecarbonaceous binder residue but essentially retain their originalparticle shapes when viewed under optical magnification. If the greenmolded preform is heated at higher temperatures (i.e. 900°-1400° C. ascarried out in U.S. Pat. Nos. 3,823,002 and 3,929,476) then some of thecontiguous refractory granules will tend to sinter together and exhibitperceptible "necking" at their contiguous points of contact when viewedunder optical magnification. Preferably necking is avoided in thepractice of this invention by heating the green molded preform attemperatures of about 700°-800° C., in order to avoid dimensionalchanges brought about by necking of the refractory granules.

A skeletal preform made by the above method will have minimal "closedporosity" (inaccessible void spaces which are wholly within the body ofthe skeleton so as to be sealed off or isolated from porosity whichcommunicates with the exterior of the preform). The major portion of thevoid space in such a preform will represent "connected porosity" (voidspace which is not sealed off from the exterior of the preform). Onlyconnected porosity can be filled by molten infiltrant.

The infiltration step 132 is carried out by placing the choseninfiltrant 133 (in solidified form) in contact with the base of theskeletal preform 131 and heating it slightly above the melting point ofthe infiltrant. The infiltrant can be copper, silver, or an alloycontaining copper or silver. Copper is the preferred infiltrant. Theamount of infiltrant is usually chosen to be slightly in excess of theamount necessary to fill the connected porosity of the skeleton (asdetermined by calculation or empirically). The surfaces of the skeletonwhich will be coincident with the working surfaces of the finalinfiltrated EDM electrode optionally can be coated with a dispersion ofzirconia in acetone in order to eliminate overwetting, i.e. "beading" or"blooming" of infiltrant on those surfaces of the preform, or anyoverwetting can be overcome by subsequent sandblasting of theinfiltrated electrode. When the melting point of the infiltrant has beenreached, the infiltrant will melt and "wick" into the interior (theconnected porosity) of the skeletal preform by capillary action, withoutthe need for applied pressure. The time necessary to infiltrate thepreform will vary depending upon the rate of heating, the grossdimensions of the preform being infiltrated, the wetting characteristicsof the infiltrant, and the diameter of the porous passages within theskeleton. However, 30 seconds to 5 minutes at a temperature slightlyabove the melting point of the infiltrant has been found sufficient toproperly infiltrate the preform. The infiltration step is preferablycarried out by supporting the preform and infiltrant metal in or on abed of alumina carried in a crucible, for example, one made of graphite,alumina, or mullite. The infiltrated preform is then cooled 134, theinfiltrated EDM electrode 135 extracted, any any excess flashing dressedoff 136.

The resulting infiltrated EDM electrode is substantially void free(i.e., it has a density at least 95% and preferably 97% or more of thetheoretical density based upon the densities of the constituents of thepreform and of the infiltrant phase). Essentially the only uninfiltratedspace in the EDM electrode will be the closed porosity of the originalpreform. The connected porosity of the original preform will beessentially completely occupied by the infiltrant phase.

The finished EDM electrode has good dimensional stability when comparedto the replicating master, that is, the major dimensions of the EDMelectrode are within 1 percent, and preferably within 0.5 percent of thecorresponding major dimensions of the replicating master. Any deviationfrom the dimensions of the replicating master is preferably of the samemagnitude and direction for each electrode prepared from a single moldso that corrections in the dimensions of a run of electrodes may be madeby alteration of the dimensions of the replicating master.

In addition, the finished EDM electrode has a high EDM cutting rate(e.g. above about 120 mm³ /minute for a tungsten-containing electrodeoperated in roughing mode). Also the finished electrode operates at alow "wear ratio" (ratio of workpiece cutting depth to longitudinalelectrode wear), preferably at a wear ratio above about 5, morepreferably at a wear ratio above about 25, and most preferably at a wearratio above about 50.

The finished EDM electrode has homogeneous density, that is, it has adensity which is uniform when compared over small volumes (e.g. 1 mm³)near the surface and within the interior of the electrode. Thiscontributes to the good dimensional stability of the EDM electrodes ofthis invention. Also, if irregularly shaped granules of carbon orrefractory are used, the finished electrode will have randomly orientedcarbon and refractory granules and randomly oriented, randomly shapedclosed porosity distributed throughout the electrode. If flaked granulesof carbon or refractory are used, the finished electrode will haveessentially randomly oriented carbon and refractory granules andessentially randomly oriented and randomly shaped closed porositydistributed throughout the electrode, with some anisotropic orientationof the flaked granules due to settling during molding.

Several other metals were examined and found not to promote copper orsilver infiltration of a carbon-containing skeletal preform as readilyas granules of tungsten, molybdenum, carbides thereof, andstoichiometric and hyperstoichiometric carbides of the other elements ofgroups IVB, VB, and VIB of the Periodic Table of the Elements. Forexample, only partial infiltration was achieved when chromium granuleswere used in place of the above refractory granules. use of tantalum ledto severe oxidation during the infiltration step, as tantalum is a veryeffective "oxygen getter" (a substance which readily forms an oxide)which removed oxygen from less stable oxides.

In FIG. 2 is shown an EDM electrode prepared as described above. Theelectrode 20 has a generally cylindrical shape with protruding flutes21, serrations 22 and a base 23. The base of the electrode is at leastone-quarter inch in thickness. The profile of the electrode lying abovethe base forms a utilitarian surface or working surface which in theparticular electrode shown corresponds to the outer profile of acommonly used electrical connector. By "utilitarian surface" is meant asurface bearing a predetermined, desired, relief pattern. The reliefpattern corresponds to the shape of a useful shaped article shaped byinjection molding or stamping or to the shape of a desired machinedcavity (e.g. a hole to be made in a hard material such as a turbineblade). The relief pattern is generally prismatic, curved (i.e. convexor concave), or both prismatic and curved in shape, and can optionallybear one or more utilitarian depressions lying within the generalprismatic or curved shape corresponding to the relief pattern, and/orcan bear one or more utilitarian projections lying outside the generalprismatic or curved shape corresponding to the relief pattern, with theutilitarian surface (including any utilitarian depressions orutilitarian projections) deviating from the predetermined desired reliefpattern by no more than one percent in any lineal dimension andpreferably by no more than 0.5 percent in any lineal dimension. Thesurface of the relief pattern is free of disturbed metal. The base ofthe electrode serves as a reference and mounting surface by means ofwhich the electrode can be fixtured in an EDM machine. In use, theelectrode is fixtured in the EDM machine, energized, and then slowlyforced into the surface of a workpiece (e.g. tool steel). The electricaldischarge passing between electrode and workpiece erodes a cavity in theworkpiece in a female shape corresponding to the electrode shape. Thecavity is fine-finished using the same or a different EDM electrode andoptionally mechanically polished. The completed cavity may be used as,for example, a die in which materials such as plastics are shaped bystamping or injection molding. The shape of such a shaped plastic partwill correspond to the shape of the utilitarian or working surface ofthe EDM electrode.

The structure of the skeletal preform may be further understood byreference to FIG. 3. FIG. 3 is a scanning electron micrograph of theinterior of an uninfiltrated shaped preform (i.e. after thermallydegrading the heat fugitive binder and before infiltration of thepreform with molten metal). The micrograph was prepared by fracturingthe preform in a Charpy Impact tester (Tinius Olsen Testing Mach. Co.)and viewing the fractured sample surface at a 45° angle of incidence anda magnification of 2100X. The preform contained 30% by volume "A-200"graphite granules, 30% by volume of a bimodal mixture of tungstengranules consisting of 65 weight % 15 micrometer mean diameter tungstenand 35 weight % 1.5 micrometer mean diameter tungsten granules, and 40%by volume void space. The dark shaded granules 31 in FIG. 3 are carbonand the lightly shaded granules 32, 33 and 33' are tungsten. Ninetypercent of the granules 31 are between 7 and 50 micrometer in meandiameter as measured by Coulter Counter. The granules 32 arerepresentative members of the 15 micrometer mean diameter tungstenfraction, and the granules 33 and 33' are representative members of the1.5 micrometer mean diameter tungsten fraction. As may be seen from aninspection of FIG. 3, some of the tungsten granules (e.g., thosedesignated 33) are interconnected with the larger carbon granules whileother tungsten granules (e.g., those designated 33') appear to beunconnected to carbon but interconnected with other tungsten granules.The various granules retain their discrete shapes. The granules aretacked or adhered together by the carbonaceous binder residue and retaintheir positions in the preform even if the sample is vigorously shaken,indicating that the various granules are firmly held in place and act asa monolith.

The metallurgical structure of an infiltrated EDM electrode may befurther understood by reference to FIG. 4, FIG. 5, and FIG. 6. Thesefigures are scanning electron micrographs of an EDM electrode of theinvention containing carbon, molybdenum and copper. A green moldedpreform was prepared by combining 70 volume percent "A-200" graphite and30 volume percent 6 micrometer mean diameter molybdenum with athermoplastic heat fugitive binder. The binder was thermally degradedand removed by heating the green molded preform in an argon atmosphere,resulting in formation of a skeletal preform containing graphite andtungsten. The preform was then infiltrated with copper. The final,shaped, electrode had uniformly distributed concentrations of carbon,molybdenum, and copper of 28 volume percent, 12 volume percent, and 60volume percent, respectively. The sample electrode was then polished bystandard metallurgical procedures.

In FIG. 4 the polished surface of the sample is shown at a magnificationof 420X. The composition of the indicated areas of FIG. 4 (and FIG. 5and FIG. 6) was determined by scanning electron microscopy (SEM). Theblack areas 41 are graphite, the dark grey areas 43 are copper andmolybdenum, and the lighter grey areas 44 are also copper and molybdenumbut are relatively richer in molybdenum than the dark grey areas 43. Thewhite fringes 42 appear to be due to electron charging.

In FIG. 5 the polished surface of the sample is shown at a magnificationof 1800X. The black areas 51 are graphite and the dark grey areas 52 arecopper and molybdenum. However, at this magnification the grey areas 53do not have the homogeneous appearance of the grey areas 43 of FIG. 4.Instead, lighter colored molybdenum granules 54 and darker copper zones55 may be seen within the dark grey area 53. The molybdenum granules 54appear to be in contact with the graphite 51. The dark grey area 53 hasa width of appropriately 3 micrometers. Some copper appears to resideadjacent the graphite 51 within dark grey area 53. The light grey area56 does not have the homogeneous appearance of the corresponding lightgrey area 44 of FIG. 4. Within the area labeled 56 may be seen lightercolored molybdenum granules 57 and darker copper zones 58. The whitefringes 52 appear to be due to electron charging.

In FIG. 6 is shown a portion of the light grey area 56 of FIG. 5, at amagnification of 3000X. The light colored molybdenum granules 61 and thedark copper zones 62 may be seen distinctly. The SEM signal within eacharea 61 and 62 gives a strong indication of molybdenum and copperrespectively. The molybdenum granules 61 generally appear to be incontiguous relationship throughout the area represented in FIG. 6.

When viewed under an optical microscope at a magnification of 750X, thegraphitic areas of the sample are a deep black, the molybdenumcontaining areas of the sample are nearly white, and the copper zones ofthe sample are reddish in color.

FIG. 4, FIG. 5 and FIG. 6 illustrate the substantially void-freecomposition of the EDM electrodes of the present invention. Thesefigures also provide evidence that the interconnected skeletal structureillustrated in the preform of FIG. 3 is preserved after the preform hasbeen infiltrated with molten metal.

The compositions of the EDM electrodes of the present invention may befurther understood by reference to FIG. 7. In FIG. 7, apex A of theternary diagram represents a 100 volume percent carbon composition, apexB represents a 100 volume percent infiltrant metal composition, and apexC represents a 100 volume percent refractory composition. The boundariesof trapezoid DEFG and the area within them represent preferredcompositions for EDM electrodes of the present invention prepared fromgreen molded preforms containing the indicated amounts of carbon andrefractory (plus a binder occupying the space later filled by theinfiltrant) which preforms are then infiltrated with the indicatedamounts of infiltrant. In general, as one crosses the boundary DE inFIG. 7 in the direction of higher carbon volume percent (i.e. frominside DEFG to the area above and to the right of it), the surface areaof carbon in comparison to the surface area of refractory becomes sogreat that the preform cannot be thoroughly infiltrated. As one crossesthe boundary EF in the direction of higher infiltrant metal volumepercent (i.e. with an eventual infiltrant metal volume percent greaterthan about 62 volume percent), the skeletal preform has such a largeunoccupied void area (connected porosity) prior to the infiltration stepthat it does not possess sufficient rigidity to survive infiltration. AnEDM electrode made with such a composition will tend to "slump" orcollapse during the infiltration step. An EDM electrode with acomposition along the ternary diagram boundary joining apex B and apex Cin FIG. 7 contains essentially no carbon and would resemble the EDMelectrodes made from pure refractory skeletal preforms described in U.S.Pat. Nos. 3,823,002 and 3,929,476. The skeletal preforms described inU.S. Pat. Nos., 3,823,002 and 3,929,476 may contain some carbonaceousresidue after the binder is thermally degraded and removed, but suchcarbon would only be present as a submicron amorphous carbon residue(i.e. as particles or a film less than about 1 micrometer in thickness),and in small quantities. The majority (i.e., greater than 50 percent) ofthe free carbon particles in a shaped, infiltrated EDM electrode of thepresent invention are greater than 1 micrometer in mean diameter. As onecrosses the boundary GD in the direction of lower infiltrant metalvolume percent (i.e. with an eventual infiltrant metal volume percentless than about 20 volume percent), it is difficult to achieve efficientpacking by vibratory molding using commercially available sizes ofcarbon and refractory granules. Compositions within DEFG but near lineGD (i.e. those with a low eventual infiltrant volume percent) generallyrequire very careful control of particle sizes, i.e. relatively largecarbon granules and much smaller refractory granules with a bimodalparticle size distribution.

The boundaries of trapezoid HIJK and the area within them in FIG. 7represent most preferred compositions for EDM electrodes of the presentinvention. The skeletal preform used to make such an electrode shouldhave a connected porosity greater than about 25 volume percent, i.e. theelectrode should have an infiltrant metal volume percent greater thanabout 25 volume percent, because attainment of connected porositiesbelow about 25 volume percent generally requires careful control ofparticle sizes in the preform matrix. Also, the preform should have aconnected porosity less than about 45 volume percent, i.e., theelectrode should have an infiltrant metal volume percent less than about45 volume percent, because use of higher amounts of infiltrant metalgenerally leads to increased shrinkage. In addition, the amount ofcarbon in the preform should be above about 15 volume percent, becausethis results in lower costs and improved EDM cutting performance.Finally, the amount of carbon and refractory in the preform should beless than that represented by boundary HI, in order to insure that thematrix skeleton can be thoroughly infiltrated. It should be emphasizedthat HIJK is based on the use of commercially available granular rawmaterials, and that its boundaries will depend somewhat upon the sizedistribution and surface area of the granular raw materials chosen.Also, it must be borne in mind that the aforementioned generalrelationship ##EQU3## should be observed in order to achieve completeinfiltration.

In general, as the volume percent of infiltrant metal is reduced,processing shrinkage of the electrode is minimized. In addition, as thevolume percent of refractory relative to carbon is reduced, EDM roughingmode cutting rate is increased, although workpiece surface finish willroughen and electrode end and corner wear will increase.

Objects and advantages of this invention are illustrated in thefollowing examples but the amounts and materials described in theexamples, and various additions and details recited therein, should notbe construed to limit the scope of this invention. Example 2 and one ofthe runs in Example 3 are comparative examples outside the scope of thepresent invention.

EXAMPLE 1

An EDM master was prepared by cutting a 5 cm long piece of 1.25 cm steelbar stock, polishing the cut ends, and boring a 1.2 mm axial holethrough the cut piece. A flexible female mold corresponding to thisshape was made using "RTV-J" silicone curable rubber (commerciallyavailable from Dow Corning). An EDM electrode containing 30 volumepercent graphite, 40 volume percent copper, and 30 volume percenttungsten was then prepared according to the present invention asfollows. A sample of "A-200" graphite granules (commercially availablefrom Union Carbide Co. of Canada) which had a mean particle size of 22micrometers with 95 volume percent greater than 7 micrometers, 5 volumepercent greater than 50 micrometers, and a maximum particle size of 74micrometers was obtained. The graphite sample appeared to have aGaussian particle size distribution as measured by Coulter Counter. Abimodal distribution of tungsten granules was prepared by combining 80wt.% of 15 micrometer tungsten granules (commercially available fromGeneral Electric Co.) with 20 wt.% 1.5 micrometer tungsten granules(commercially available from General Electric Co.). Twenty seven gramsof the above graphite granules and 230 g of the above bimodal tungstengranule mixture were then combined by dry mixing to form a powdermixture. The dry granules were poured onto a heated rubber mill andcombined with 15 g of binder ("Emerest 2642", a polyethylene glycoldistearate with number average molecular weight of 400, commerciallyavailable from Emery Industries). The mill rolls were maintained at atemperature of about 80° C. This mixture was milled for about 15 minutesand resulted in a thixotropic warm powder-binder mixture.

The warm powder-binder mixture and the flexible mold were brought to 65°C. by storing in a 65° C. oven for about 15 minutes. The powder-bindermixture was then flowed into the flexible mold by vibratory means. Themixture was deaired for 15 minutes with continued vibration in alaboratory vacuum chamber operated at 1 torr. The mold and contents werethen cooled to 0° C. in a freezer and the hardened, green molded preformsubsequently extracted from the rubber mold cavity using vacuum.

The green molded preform was placed in a supporting bed of powderedalumina and heated in a resistance heated box furnace with a dynamicargon atmosphere. A temperature of approximately 400° C. was sufficientto volatilize and thermally degrade most of the binder. Heating wasdiscontinued when the temperature reached 780° C., at which point thebinder had essentially completely degraded and the skeletal particles ofcarbon and tungsten had become tacked together, forming a skeletalpreform.

The shaped skeletal preform was removed from the furnace after it hadcooled to room temperature. An acetone dispersion of zirconia (50% byvolume) was applied to all but one surface (the base) of the shapedpreform in order to prevent the infiltrant metal from overwetting theworking surfaces. The base of the preform was then placed adjacent 135 gof solid copper on a bed of alumina in an open graphite crucible in amolybdenum wound electrical resistance furnace. The furnace wasevacuated to 0.1 torr and refilled with argon to atmospheric pressureand maintained at a flow rate of 0.47 liters/second. The furnace washeated to 1120° C. and held at that temperature for 45 minutes in orderto carry out infiltration of the skeletal preform by copper infiltrant.The furnace was then turned off and allowed to cool normally. Theinfiltrated EDM electrode was removed from the crucible and excessflashing dressed off.

Dimensional change during processing was measured by comparing the 1.25cm square end of the master shape to the final molded electrode alongtwo perpendicular axes and averaging the change in dimension. Theelectrode was tested for cutting preformance using a Charmilles D-10 EDMmachine in two cutting modes (finishing and roughing). EDM settings were3 apere maximum, 2 microseconds on, 3 microseconds off for finish mode,and 25 ampere maximum, 25 microseconds on, 4 microseconds off for roughmode. The wear ratio was determined by comparing the ratio of work piececutting depth to electrode longitudinal wear. For consistency, constantamounts of material were removed for this and each of the followingelectrode tests, i.e. 0.076 mm depth for finish mode and 2.5 mm forroughing mode. The finished electrode made by this process exhibited thefollowing performance:

    ______________________________________                                        Dimensional Change    -0.6%                                                   Finish cutting rate   1.7 mm.sup.3 /min.                                      Finish wear ratio     29.9                                                    Rough cutting rate    162 mm.sup.3 /min.                                      Rough wear ratio      20.5                                                    ______________________________________                                    

The fraction (see formula I, above) ##EQU4## in this example was##EQU5##

EXAMPLE 2

An attempt was made to prepare an EDM electrode using the method ofExample 1 but with 40 to 200 micrometer graphite (Asbury "1264",commercially available from Asbury Graphite Mills) and 40 to 200micrometer tungsten ("M-70", commercially available from GTE Sylvania).Ambient pressure infiltration did not take place.

The fraction I in this example was ##EQU6##

EXAMPLE 3

The comparative test preformed in Examples 1 and 2 was repeated using alower graphite volume percentage. A preform containing 15 volume percentgraphite granules ("A-200", as used in Example 1) and 41 volume percenttungsten granules (a bimodal mixture as used in Example 1) wasinfiltrated by ambient pressure infiltration. A preform containing 15volume percent graphite granules (Asbury "1264", as used in Example (2)and 41 volume percent tungsten granules ("M-70", as used in Example 2)could not be infiltrated by ambient pressure infiltration. Thecalculations of Example 1 and Example 2 were repeated. For the samplewhich infiltrated by ambient pressure infiltration, the fraction I was:##EQU7## For the sample which failed to infiltrate by ambient pressureinfiltration, the fraction I was: ##EQU8##

EXAMPLES 4-27

Using the method of Example 1, several EDM electrode compositions wereprepared and evaluated for performance. A thermoset binder was used inExample 15. It was prepared by combining 28 volume % "Epon" 825 epoxyresin (Shell Chemical Co.), 12 volume % "Epon" F-2 amine curing agent,30 volume % "Carbowax" 400 (a polyethylene glycol with a number averagemolecular weight of 400, commercially available from Union Carbide Co.of Canada), and 30 volume % 1,3-butanediol. The preform of Example 15was cured by heating to 38° C. for 18 hours. The results for Examples 4to 27 are set out below in tabular form:

                                      TABLE III                                   __________________________________________________________________________       Preform composition                Cutting rate for                           (volume % excluding                                                                      Final electrode composition                                                                    Dimensional                                                                          12.5 × 12.5 mm                    Ex.                                                                              void space)                                                                              (volume %)       change electrode (mm.sup.3 /min)                                                                Wear ratio                   no.                                                                              C.sup.a                                                                          W  Other.sup.b                                                                        C.sup.a                                                                          W  Other.sup.c                                                                       Cu Alloy                                                                             (%)    Finish.sup.d                                                                        Rough.sup.e                                                                        Finish.sup.d                                                                      Rough.sup.e              __________________________________________________________________________     4    100.sup.f  70     30     -0.7   2.1   100  7.5 65.6                      5  7 93.sup.f                                                                               5 64.7   30.3   -0.7   2.3   120  5.8 32.9                      6  7 93.sup.f                                                                               5 64.7      30.3.sup.g                                                                        -0.6   3.4   119  9.5 32.9                      7 30 70.sup.f                                                                              17.7                                                                             41.3   41     -2.0   3.8   174.sup.h                                                                          5.8  9.8.sup.h                8 30 70.sup.f                                                                              17.7                                                                             41.3      41.sup.i                                                                          -2.0   4.0   194.sup.h                                                                          3.8 19.7.sup.h                9 40 60.sup.f                                                                              23.5                                                                             35.2   41.3   -3.3   3.6   174  15.2                                                                              19.7                     10 40 60.sup.f                                                                              23.5                                                                             35.2      41.3.sup.j                                                                        -2.0   4.2   167  1.5  9.0                     11 50 50.sup.k                                                                              25 25     50     0      2.5   169.sup.h                                                                          1.5 14.0.sup.h               12 50 50.sup.k                                                                              25 25        50.sup.l                                                                          +0.2   2.6   127.sup.h                                                                          2.7  8.2.sup.h               13 50 50.sup.m                                                                              25 25     50     0      4.6   176.sup.h                                                                          3.8  6.6                     14 50 50.sup.n                                                                              25 25     50     +0.2   3.0   149.sup.h                                                                          3.8 21.0                     15.sup.p                                                                         50.sup.q                                                                         50.sup.f                                                                              30.sup.q                                                                         30     40     -0.8   --    183.sup.r                                                                          --  100.sup.r                16 80 20.sup.k                                                                              31.3                                                                              7.8   60.9   -4.1   5.6   175  3.3  7.9                     17       100.sup.s  50.sup.t                                                                          50     0.9    1.4    82  3.3 37.9                     18 15    85.sup.s                                                                            6.2  37.2.sup.t                                                                        56.6   -1.1   3.3   111  9.5 12.3                     19 60.sup.u                                                                            40.sup.s                                                                           33    22.sup.t                                                                          45     -0.7   6.1   165  9.5 12.3                     20 65.sup.v                                                                            35.sup.s                                                                           39.9  21.5.sup.t                                                                        38.6   -0.5   4.4   131  3.0  9.4                     21 65    35.sup.w                                                                           33.2  17.9.sup.t                                                                        48.9   --     --    161  --  12.3                     22       100.sup.x                                                                          26.2  17.5.sup.t                                                                        56.3   -4.1   4.2    80  3.0 10.4                     23 60    40.sup.y                                                                           29.1  19.4.sup.y                                                                        51.6   -3.3   3.8   149  3.0 10.4                     24 50.sup.z                                                                            50.sup.aa                                                                          30.sup.z                                                                            30.sup.aa                                                                         40     -0.6   3.0   183  6.0 24.6                     25 30.sup.k                                                                            70.sup.bb                                                                          30.4                                                                             15.6                                                                              5.8                                                                              48.2   0      4.2   167  3.0 14.0                     26 30.sup.k                                                                            70.sup.bb                                                                          30.4                                                                             15.6                                                                              5.8                                                                              48.2.sup.g                                                                           0      3.8   129  25.3                                                                              13.1                     27       26.sup.s /74.sup.bb                                                                30    20  50     -0.7   4.6   121  4.2  7.3                     __________________________________________________________________________     .sup.a Carbon was "A200" graphite granules (Union Carbide Co. of Canada),     unless otherwise indicated                                                    .sup.b Either Mo, WC, Mo.sub.2 C, or molybdenum coated graphite               .sup.c Either Mo, WC, or Mo.sub.2 C                                           .sup.d EDM settings were 3 ampere maximum, 2 microseconds on, 3               microseconds off, unless otherwise indicated                                  .sup.e EDM settings were 25 ampere maximum, 25 microseconds on, 4             microseconds off, unless otherwise indicated                                  .sup.f Bimodal distribution consisting of 65 wt. % 15 micrometer W and 35     wt. % 1.5 micrometer W                                                        .sup.g 80% Cu, 20% Sn                                                         .sup.h EDM settings were 25 ampere maximum, 200 microseconds on, 25           microseconds off                                                              .sup.i 90% Cu, 10% Ag                                                         .sup.j 75% Cu, 15% Ni, 10% Sn                                                 .sup.k 0.8 micrometer W                                                       .sup.l 60% Cu, 20% Ni, 20% Mn ("chase alloy")                                 .sup.m 1.5 micrometer W                                                       .sup.n 8 micrometer W                                                         .sup.p Thermoset binder prepared from 28 vol. % "Epon" 825 (Shell Chemica     Co.), 12 vol. % "Epon" F2 (Shell Chemical Co.), 30 vol. % "Carbowax" 400      (Union Carbide Co.), and 30 vol. % 1,3butanediol                              .sup.q "1264" graphite granules (Asbury Graphite Mills)                       .sup.r EDM settings were 25 ampere maximum, 400 microseconds on, 25           microseconds off                                                              .sup.s 6 micrometer Mo                                                        .sup.t Mo                                                                     .sup.u No. 2 Natural Flake Graphite (Dixon Carbon Co.)                        .sup. v "Versar" spherical carbon beads (Versar Carbon Co.)                   .sup.w "SD252" molybdenum 0.8 micrometer particles which form aggregates      approximately 8 micrometers in size (GTE Sylvania Inc.)                       .sup.x 75/25 wt. % Mo coated graphite (Sherritt Gordon Mines, Ltd.,)          .sup.y Bimodal distribution consisting of 65 wt. % WC ("SC60", General        Electric Co.) and 35 wt. % WC ("WCOOPL, WahChang Div. of Teledyne Corp.)      .sup.z "4349" graphite granules (Asbury Graphite Mills)                       .sup.aa Mo.sub.2 C (Ventron Corp.)                                            .sup.bb 50/50 wt. % Mo coated graphite (Sherritt Gordon Mines, Ltd.)     

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention and the latter should not be restricted to that setforth herein for illustrative purposes.

What is claimed is:
 1. A shaped, electrical discharge machiningelectrode, consisting essentially of:(1) a monolithic skeletonconsisting essentially of (a) granules of carbon and (b) granules of arefractory material selected from the group consisting of tungsten,molybdenum, carbides of these two metals and stoichiometric andhyperstoichiometric carbides of the other elements of groups IVB, VB,and VIB of the Periodic Table of the Elements, the majority of saidcarbon granules being greater than about one micrometer in meandiameter, said refractory material being wettable by molten copper orsilver, said carbon granules and said refractory granules beinginterconnected at their contiguous points of contact; and (2) acontinuous phase consisting essentially of copper, silver, or alloyscontaining copper or silver, and occupying the connected porosity insaid skeleton;wherein the volume percent of said carbon, saidrefractory, and said continuous phase are on or within the boundariesDEFG of FIG. 7, and the fraction expressed by the formula ##EQU9##wherein C is said carbon, R is said refractory, the volume percent termsare the fraction of said electrode occupied by C or R, and the surfacearea terms are the surface areas in m² /g of said carbon or refractorygranules measured before said carbon or refractory granules are used inmaking said electrode, is less than about 75, said electrode therebybeing a monolithic structure consisting essentially of two intermeshedmatrices, being substantially free of voids, and being substantiallyfree of materials which are soluble in said continuous phase andreactive with carbon.
 2. An electrical discharge machining electrodeaccording to claim 1, wherein said electrode has homogeneous density andsaid carbon granules and said refractory granules have essentiallyrandom orientation.
 3. An electrical discharge machining electrodeaccording to claim 1, wherein said carbon granules are amorphous carbon.4. An electrical discharge machining electrode according to claim 1,wherein said carbon granules are graphite.
 5. An electrical dischargemachining electrode according to claim 1, wherein said carbon granuleshave a surface area of less than about 15 m² /g.
 6. An electricaldischarge machining electrode according to claim 1, wherein said carbongranules have a surface area of less than about 7 m² /g.
 7. Anelectrical discharge machining electrode according to claim 1, whereinsaid refractory is tungsten.
 8. An electrical discharge machiningelectrode according to claim 1, wherein said refractory is molybdenum.9. An electrical discharge machining electrode according to claim 1,wherein said refractory is a stoichiometric or hyperstoichiometriccarbide of an element selected from groups IVB, VB, and VIB or thePeriodic Table of the Elements.
 10. An electrical discharge machiningelectrode according to claim 1, wherein said refractory is tungstencarbide.
 11. An electrical discharge machining electrode according toclaim 1, wherein said refractory is molybdenum carbide.
 12. Anelectrical discharge machining electrode according to claim 1, whereinsaid refractory is molybdenum precipitated onto a carbon nucleatingagent.
 13. An electrical discharge machining electrode according toclaim 1, wherein said refractory granules have a bimodal distribution ofparticle sizes.
 14. An electrical discharge machining electrodeaccording to claim 1, wherein the volume percent of said carbon, saidrefractory, and said continuous phase are on or within the boundariesHIJK of FIG.
 7. 15. A precision molded electrical discharge machiningelectrode, consisting essentially of(1) a skeleton consistingessentially of (a) granules of graphite and (b) granules of tungsten,the majority of said graphite granules being greater than about 1micrometer in mean diameter, said graphite granules and said tungstengranules being interconnected at their contiguous points of contact; and(2) a continuous phase consisting essentially of copper occupying theconnected porosity in said skeleton;wherein the volume percent of saidgraphite, said tungsten, and said continuous phase are on or withinboundaries DEFG of FIG. 7, and the fraction expressed by the formula##EQU10## wherein C is said graphite, R is said tungsten, the volumepercent terms are the fraction of said electrode occupied by C or R, andthe surface area terms are the surface areas in m² /g of said graphiteor tungsten granules measured before said graphite or tungsten granulesare used in making said electrode, is less than about 75, said electrodethereby consisting essentially of two intermeshed matrices beingsubstantially free of voids substantially free of materials which aresoluble in said continuous phase and reactive with carbon.
 16. Anelectrical discharge machining electrode according to claim 15, whereinthere is no perceptible necking between the contiguous granules of saidtungsten.
 17. An EDM electrode according to claim 15, wherein the volumepercent of said graphite, said tungsten, and said continuous phase areon or within the boundaries HIJK of FIG.
 7. 18. A skeletal article,consisting essentially of granules of carbon and granules of arefractory material selected from the group consisting of tungsten,molybdenum, carbides thereof, and stoichiometric and hyperstoichiometriccarbides of the other elements of groups IVB, VB, and VIB of thePeriodic Table of the Elements, the majority of said carbon granulesbeing greater than about one micrometer in size, said refractorymaterial being wettable by molten copper or silver without use of awetting promoting metal, and said carbon granules and said refractorygranules being interconnected at their contiguous points of contact byan amorphous carbon residue less than one micrometer in thickness.
 19. Askeletal article, consisting essentially of:(a) granules of carbon, (b)granules of a refractory material selected from the group consisting oftungsten, molybdenum, carbides thereof, and stoichiometric andhyperstoichiometric carbides of the other elements of groups IVB, VB,and VIB of the Periodic Table of the Elements, and (c) granules ofcopper, silver, or an alloy containing copper or silver,the majority ofsaid carbon granules being greater than about one micrometer in size,said refractory material being wettable by molten copper or silverwithout use of a wetting promoting metal, and said granules (a), (b),and (c) being interconnected at their contiguous points of contact by asubmicron amorphous carbon residue.